drm/i915: Add dev to ppgtt

Some subsequent commits will need to know what generation we're running
on to do different pte encoding for the ppgtt. Since it's not much
hassle or overhead to store it in the ppgtt structure, do that.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>